It is often desirable in the microelectronics industry to be able to implement physical solutions in as little time as possible for coping with new market opportunities and/or emerging problems. Computer-provided simulations are often used to predict the behaviors of to-be-manufactured integrated circuits or other like systems. This is preferably done before the integrated circuits are finalized so that design errors may be corrected and design parameters may be optimized prior to mass production. It is well known, for example, to use computers and software simulators such as SPICE™ for obtaining fairly accurate predictions of the analog behaviors of complex circuitry.
SPICE-like simulations may provide fairly accurate predictions of how corresponding circuits will behave when actually built. The predictions are preferably made not only for individual sub-circuit but also for whole systems (e.g., whole integrated circuits) so that system-wide problems relating to noise and the like may be uncovered and dealt with. FIG. 1A illustrates a general process flow of a SPICE-like simulation. An analog integrated circuit under simulation is often represented in the form of a netlist description 102. A netlist is a circuit description of the analog circuit to be simulated written in a SPICE-like language. SPICE netlists are pure structural languages with simulation control statements. Other language like Verilog-A™ has the capability to include behavioral constructs. The structural netlist of SPICE together with a predefined set of circuit components of the analog integrated circuit may be represented in the form of a matrix 104 in accordance with certain circuit modeling methodologies (which is not a concern of the present invention). The number of non-linear differential equations ranges from 1 to n. There is a corresponding number of input vectors to be operated by the linear equation. The set of input vectors are shown as {I1, I2, . . . In} 106. Next, the linear matrix is computed with the set of input vectors to generate a set of solution vectors {V1, V2, . . . Vn} 108. The computation is repeated until the set of solutions converge. The set of solutions may be then displayed in the form of waveforms, measurements, or checks 110 on a computer screen for engineers to inspect the simulation results.
However, SPICE-like simulation of a whole system becomes more difficult and problematic as the industry continues its relentless trek of scaling down to smaller and smaller device geometries and of cramming more interconnected components into the system. An example of such down scaling is the recent shift from micron-sized channels toward deep submicron sized transistor channel lengths. Because of the smaller device geometries, circuit designers are able to cram exponentially larger numbers of circuit components (e.g., transistors, diodes, capacitors) into a given integrated circuit (IC), and therefore increases the matrix size to a complexity which may not be solved in a desired time frame.
FIG. 1B illustrates a matrix representation of the partitions of a complete system of FIG. 1A. As shown in FIG. 1B, the matrix 104 may include groups of sub-matrices, namely matrices for partitions 1 through L 120, each sub-matrix 120 includes a set of equations representing the corresponding circuit partition. In general, the task of an analog simulation engine is to solve a system of non-linear differential equations. One of the most common approaches is to form a set of linear equations in a matrix and to solve them with a matrix solver. Although there are many optimizations and improvements available for matrix solvers to improve computation efficiency, the complexity of the algorithms is quite high that performance suffers as the matrix size increases. The increase in matrix size is due to the fact that identifying partitions is not obvious since the electrical characteristics of the partitions are rarely completely disconnected.
Each sub-matrix 120 may further include matrices representing individual circuits in the partition. As the number of circuit elements in an integrated circuit continues to increase, the size of the matrix 104 may become exponentially large. This results in long simulation cycles and therefore long product development cycles. Another problem with solving the circuit represented by the matrix 104 is that it does not concern about the extend of the communications 130 happening between the partitions during simulation. For example, if there is not much communication between the partitions, that is, the partitions are loosely coupled, then it would be more efficient to compute the individual partitions with their corresponding block matrices instead of computing the partitions together in one large matrix.
As the product development cycles continue to shorten, there is a need for the makers of SPICE-like simulators to come up with new ways to quickly and accurately predict the system-wide behaviors of these exponentially more dense and more complex analog integrated circuit designs. Representing analog integrated circuits in a hierarchical data structure is one of the known methods in addressing the problem stated. In designs that employ hierarchical data structure, opportunities exist for taking advantage of the redundant behaviors that are sometimes exhibited by structurally redundant subcomponents of the hierarchical structure. Examples of integrated circuits which tend to exhibit extensive hierarchical data structure include high-density memory chips such as SRAMs, DRAMs, EEPROMs, etc. Parallel data processing systems and telecommunication systems also tend to have hierarchical structures with redundant subcomponents. FIG. 2 illustrates a repetitive structure of a typical memory circuit. As shown in FIG. 2, the memory circuit includes 64,000 repetitive columns 202, each of the column includes 512 repetitive rows and each row is represented by a row branch circuit 204 which in turn calls to a leaf circuit 206. All 512 repetitive rows in the column are connected together through a node 208, and each node is driven by a sense amplifier 210.
A problem with the current methodologies in simulating the type of repetitive circuit structure of FIG. 2 is that such methodologies do not take advantage of the fact that loads of the repetitive circuit are substantially the same in most situations and only a few loads change dynamically during simulation. For instance, there are 64,000 loads representing the 64,000 columns of memory, and some of the columns have substantially the same loads during simulation.
Yet another problem with the current methodologies in simulating the type of repetitive circuit structure of FIG. 2 is that the load of the leaf circuit 206 in each row is referenced individually. When there is a change at the input port of one of the leaf circuits 206, all loads of the leaf circuits in the column need to be communicated through the hierarchical data structure to the driver sense amplifier circuit. This would result in at least 512 hierarchical traversals in communicating the changes of signal conditions at an input port of a leaf circuit in the column.
FIG. 3 illustrates various representations of the memory circuit of FIG. 2. FIG. 3A illustrates a netlist representation of the memory circuit of FIG. 2 in a machine readable language. FIG. 3B illustrates a physical representation of the same memory circuit of FIG. 2. The physical view represents the potential circuit layout, which illustrates the instantiations and duplication of components throughout the hierarchy. The top level circuit is called the Root which includes substantially the same sub-circuits from C1 to C64000, each representing a column of the memory circuit. Each column includes substantially the same row circuits from R1 to R512, each representing a row of the memory circuit. Each row includes a substantially the same leaf circuit. FIG. 3C illustrates a flatten representation of the same memory circuit of FIG. 2. In a flattened representation, all leaf circuits are illustrated on the same level. Hierarchy is collapsed and is represented by different instance names. For instance, the leaf circuits in the block C1 are called C1.R1, C1.R2, C1.R3, and so on. Other leaf circuits in the other column blocks are named in a similar manner. FIG. 3D illustrates a hierarchical representation of the same memory circuit of FIG. 2. The hierarchical representation illustrates graphically the individual components and is capable of combining duplicated representations of the same structural component. At the Root level 308, there are 64000 substantially the same instances of the same block, namely C1 to C64000. At the block level 310, there are 512 substantially the same instances of the leaf circuit, namely R1 to R512. At the circuit level 312, there is only one instance of the leaf circuit.
Rather than instantiating and simulating all the redundant subcomponents of a system individually, it is possible to pick one of many, alike subcomponents and to consider the picked subcomponent as having a behavior that is representative of the behaviors of the others. The predicted behavior results obtained for the representative one subcomponent may be duplicated for the others without repeating the computational intensive simulation processing for each of the redundant subcomponents. As such, a significant amount of computing time and computational resources may be saved if redundant subcomponents may be identified and their behaviors may be predicted in this way. As described in FIG. 6, the hierarchical data structure has the problem of communicating changes of signal conditions between circuits in different branches of the hierarchical data structure.
FIG. 4 illustrates a process for creating a simulation database for a circuit represented in hierarchical data structure. The process starts with a SPICE netlist 402. Next, a parser creates a primitive database 404 from the netlist 402. The primitive database 404 contains components such as sub-circuits and primitives of the SPICE netlist 402. Since these components are usually parameterized, a compilation step is performed on the primitive database to annotate the primitives with their actual parameters and creates a set of actual instances. These instances are stored in the simulation database 406 having one element per unique parameter set.
Prior to simulation, the process of static partitioning is performed on the simulation database 406. This process identifies the topology information of a circuit, including all the components in a circuit, the input and output ports of each circuit component, and the connections among the circuit components through their corresponding input and output ports. During simulation, the process of dynamic partitioning is performed on the simulation database.
A problem with the simulation database 406 is that it does not distinguish between the static information and the dynamic information necessary for simulating a particular partition of the circuit during certain period of the simulation. Hence, the simulator carries a large amount of information for the whole circuit. In addition, during simulation, when there is a need to duplicate a partition of a circuit, the whole topology as well as the dynamic state of the partition is duplicated, which results in more memory usage and potentially more computations. Another disadvantage of combining both static and dynamic information of a circuit into a single simulation database is that whenever a partition of the circuit is duplicated dynamically during the simulation, the simulation database is modified, including the static information of the circuit, which may not need to be modified.
FIG. 5 illustrates a method for performing the static partitioning step in FIG. 4 for building the simulation database. First, the ground nodes of the circuit are marked. In the exemplary circuit, there are two ground nodes 502 and both are marked with the label 1. Next, the power nodes driven by voltage sources are marked. There is one power node 504 and it is marked with the label 2. Then, the channel connection nodes which connect two transistors together are marked. There are four channel connection nodes 506 and they are marked with the label 3. The input ports 508 are marked with the label I, the output ports 510 are marked with the label O and the internal nodes are marked with the label N. Note that a channel connection node may be an output port in certain instances, and may be an internal node in some other instances. The gate terminal of a transistor has typically high impedance, and thus separates the coupling between one group of circuits from another group of circuits. In this exemplary circuit, the gate terminal of the second partition 514 separates the output signal from the first partition 512. This separation forms a natural boundary for the first and second partitions of the exemplary circuit.
As shown in FIG. 5, there are two partitions created for this circuit. During simulation, a simulation solution of partition 1 may need to be communicated to partition 2. Similarly, the changes at the input of partition 2 may also affect the load seen by partition 1, this information too may need to be communicated back to partition 1. It is possible that one output port may have many loads, that is, one output port may have many receivers in different partitions.
FIG. 6 illustrates a method of communicating a simulation solution in a hierarchical data structure. A simulation solution may be communicated in both forward direction and backward directions. A five-level hierarchical data structure is shown here for purpose of illustration. In reality, the number of levels of hierarchy depends on the actual circuit design. In FIG. 6, when the signal conditions at the output port of the inverter leaf circuit 620 change, this information may need to be propagated to all receivers of the inverter, one of such receivers is the next leaf circuit 622. In a hierarchical data structure, a forward propagation is communicated to as many potential receivers as possible. The information is propagated up the hierarchy to the anchor node 602 and from there the information is propagated down one level at a time until the information reaches all the receivers. At each intermediate level, the information received at that level are synchronized before passing to the next level.
For reverse propagation, when the signal conditions change at the input port of the inverter leaf circuit 620, this change affects the load of the output driver which drives the leaf circuit 620. This is because the impedance, capacitance and the current consumed by the receiver leaf circuit changes. In this case, the driver output port may need to be informed of the change in load of leaf circuit 620. As shown by the dotted arrows, reverse propagation passes information directly to the root circuit 602 and from there the information is propagated down one level at a time. From the root circuit 602, the simulator finds all the branches that need to receive the information and propagates down the hierarchy one level at a time. At each intermediate level, the information received at that level are synchronized before passing to the next level.
A problem with the method of communicating changes of signal conditions as illustrated in FIG. 6 is that in order to communicate such information from one leaf circuit to other leaf circuits, the information needs to traverse many levels of the hierarchical data structure before reaching its destination. At each hierarchical level, information needs to be synchronized before it may be transmitted to the next level. Therefore, the method of passing information through the hierarchies and synchronizing at each intermediate level result in lower simulation performance.